我知道这个问题已经被问了很多了，我发现所有类似的问题仍然无法解决，所以请我知道错误在哪里以及如何纠正它？
<html>
<body>
<?php
$num_of_calls = 0;
function numberedHeading($txt){
global $num_of_calls;
$num_of_calls++;
echo "<h1>".$num_of_calls." ".$txt."</h1>";
}
numberedHeading("Widgets");
echo "<p>We build a fine range of widgets.</p>";
numberedHeading("Doodads");
echo "<p>Finest in the world.</p>";
?>
</body>
</html>